1. Field of the Invention
The present invention relates to a laser oscillating apparatus and particularly to a laser oscillating apparatus designed to stabilize the pointing of a laser beam.
2. Description of the Background Art
FIGS. 21 and 22 are perspective views illustrating the arrangement of, for example, a laser oscillating apparatus of the background art disclosed in Japanese Patent Disclosure Publication No. 254684 of 1985. Referring to FIG. 22, the numeral 10 indicates an enclosure for enclosing a laser medium gas, 4 a pair of discharge electrodes, 8 a heat exchanger, 6 a blower, 26, 28 and 30 total reflectors, 32 a partial reflector, 12a first laser beam reflecting unit including the partial reflector 32 and total reflector 28, 12b second laser beam reflecting unit including the total reflectors 26 and 30, and 2 a laser beam. FIG. 23 shows a cross section of the first laser beam reflecting unit 12a, wherein 14 and 15 indicate apertures disposed in aperture member 95 immediately before the partial reflector 32 and total reflector 28, respectively, 36 an optical base for holding the first laser beam reflecting unit 12a, 44 a connecting rod for connecting the optical base 36 and an optical base for the second laser beam reflecting unit 12b, 54 bellows on which the enclosure 10 and optical base 36 are installed to maintain vacuum and airtightness, 38 an optical board installed on the optical base 36, and 40 and 42 adjusting plates mounted with the total reflector 28 and partial reflector 32 for adjusting the angles of the total reflector 28 and partial reflector 32, respectively.
FIGS. 24 and 25 illustrate the optical board 38 and an angle adjusting mechanism of the adjusting plate 40 installed on the optical board 38. FIG. 24 is a side view and FIG. 25 is a sectional view taken along the plane 25--25 of FIG. 24. In FIGS. 24 and 25, 46 indicates adjusting screws for adjusting the angle of the adjusting plate 40, 47 a threaded portion where the adjusting screw 46 is threaded into the optical board 38, 48 an O ring for vacuum-sealing and simultaneously holding the adjusting screw 46 rotatably against the optical board 38, 49 a receiving member provided on the adjusting plate 40 to make contact with the end of the adjusting screw 46, 55 a spring disposed to pull the adjusting plate 40 toward the optical board 38, 50 a support provided on the adjusting plate 40, 56 coolant flowing in the adjusting plate 40, 58 tubes in which the coolant 56 flows, 51a and 51b holes formed in the optical board 38 to permit a flow of the coolant 56, and 52 joints for connecting the tubes 58, optical board 38 and adjusting plate 40.
The operation of the laser oscillating apparatus arranged as described above will now be described.
FIG. 26 is a vertical sectional diagrammatic view in the longitudinal direction of the oscillating apparatus including the resonator light paths of the laser oscillating apparatus described in FIG. 22. Referring to FIGS. 26 and 22, the pair of discharge electrodes 4 for generating discharge and exciting the laser medium gas, the blower 6 for circulating the laser medium gas, and the heat exchanger 8 for cooling the laser medium gas are disposed in the enclosure 10 as described previously, and the laser medium gas passes between the pair of discharge electrodes 4 and is excited to be ready for laser oscillation. The laser medium gas then enters the heat exchanger 8, is cooled there, passes through the blower 6, and circulates in the direction of an arrow A. In the meantime, three resonator light paths pass an excitation area 18, where the laser medium gas is excited by discharge, in a Z pattern. The three parts are formed by resonator mirrors, comprising the total reflectors 26, 28 and 30 and the partial reflector 32 disposed in the longitudinal direction of the enclosure 10.
The laser beam 2 reflected by the total reflector 26 passes a first optical axis 20 and reaches the total reflector 28. Since the total reflector 28 is disposed slanting downward at an angle of .theta. relative to the first optical axis 20, the laser beam 2 passes a second optical axis 22 slanting downward at an angle of 2.theta. with respect to the first optical axis 20 and reaches the total reflector 30. Since the total reflector 30 is disposed slanting upward at an angle of .theta. relative to the first optical axis 20, the laser beam 2 passes a third optical axis 24 parallel with the first optical axis 20 and reaches the partial reflector 32. Part of the laser beam 2 reaching the partial reflector 32 is output intact to the outside and the rest thereof returns to the total reflector 26 on an opposite route to the above. With this process repeated, the laser beam 2 is amplified while passing the excitation area 18 repeatedly, and at a proper energy level is output to the outside from the partial reflector 32.
The angle adjusting mechanism of the total reflector 28 will now be described with reference to FIGS. 24 and 25. The angle of the total reflector 28 installed on the adjusting plate 40 is adjusted by the adjustment of the angle of the adjusting plate 40. Since the adjusting plate 40 is pulled by the spring 55 toward the optical board 38 and is pushed back by the support 50 and two adjusting screws 46, the angle of the adjusting plate 40 is determined by a relationship between the length of the support 50 and the projection length of the adjusting screws 46 from the optical board 38. Namely, by turning the adjusting screws 46, the projection length thereof from the optical board 38 is changed, allowing the angle of the adjusting plate 40 to be adjusted up and down and/or side to side.
In the meantime, on receiving the laser beam 2, the total reflector 28 having a certain absorption factor for the laser beam 2 absorbs part of the laser beam 2 and generates heat. The total reflector 28 is indirectly cooled by the coolant 56 which cools the adjusting plate 40. The coolant 56 enters from the outside into the tube 58, passes the hole 51a in the optical board 38, then passes a hole formed in the adjusting plate 40, re-enters the hole 51b in the optical board 38, and finally goes out.
In terms of temperature, while the optical board 38 making direct contact with the coolant 56 depends on the coolant temperature, the optical base 36 depends on the ambient temperature. Accordingly, when a difference occurs between the coolant temperature and ambient temperature, a temperature difference is also produced between the optical board 38 and optical base 36. Since a linear expansion coefficient is different between the optical board 38 and optical base 36, thermal stress is generated therebetween. Generally, the optical base 36 is built very sturdily to support most of the components of the first or second laser beam reflecting means 12a or 12b. Hence, the thermal stress produced between the optical board 38 and optical base 36 causes distortion in the optical board 38.
When distortion occurs in the optical board 38 as described above, the angles of the adjusting plates 40 and 42 change, which forces the angles of the total reflector 28 and partial reflector 32 to change and the optical axes of the laser beam in the resonator to be misaligned, deteriorating the pointing (positional accuracy) stability of the laser beam.
The second optical axis 22 in FIG. 26 and the total reflectors 28 and 30 located at both ends of the second optical axis 22 will now be noted. Since the total reflector 28 slants downward at the angle of .theta. relative to the first optical axis 20 as described previously, the laser beam 2 reaches the total reflector 30 through the second optical axis 22 slanting downward at the angle of 2.theta. relative to the first optical axis 20. Since the total reflector 30 slants upward at the angle of .theta. relative to the first optical axis 20, the laser beam 2 passes the third optical axis 24 parallel to the first optical axis 20. In other words, the second optical axis 22 slants at the angle of 2.theta. relative to the first and third optical axes 20 and 24, and the reflective surfaces of the total reflectors 28 and 30 located at both ends of the second optical axis 22 are parallel to each other, slanting at the angle of .theta. relative to the first and third optical axes 20 and 24, respectively.
In such an arrangement, portions 34 and 35 where the reflective surfaces of the total reflectors 28 and 30 are opposed to each other occur in the openings of the apertures 15 and 16 as shown in FIG. 27. This causes a probability that parasitic oscillation 36 will be generated between portions 34 and 35 where the total reflectors 28 and 30 are opposed to each other, in addition to the ordinary laser oscillation. If this parasitic oscillation takes place, the beam mode of the laser beam 2 emitted by the laser oscillating apparatus will be faulty or the stability of the beam mode will be worsen.
The magnitude of beam mode control is determined by a ratio .PHI./.omega. (hereinafter referred to as the "beam mode control factor"); .omega. is 1/e.sup.2 radius in a single mode determined by resonator mirror curvature and resonator length (light path length from the total reflector 26 to the partial reflector 32), where e indicates the base of natural logarithm and .PHI. is an aperture diameter. In the single mode, an aperture having an approximately 3.1 to 3.4 beam mode control factor .PHI./.omega. is often selected as a controlling aperture value. Thus, as the beam mode control factor .PHI./.omega. becomes smaller, the degree of controlling the beam mode becomes higher.
In the conventional laser oscillating apparatus, the beam mode control factor .PHI./.omega. of one of four apertures 14 to 17 as shown in FIG. 18 is made lower than those of the other three apertures to prevent the occurrence of the parasitic oscillation 36. This is because the adjustment of the resonator mirrors will be difficult if the mode is controlled in many places. For instance, the apertures 14, 16 and 17 are set to the beam mode control factor .PHI./.omega. of 3.6, and only the aperture 15 is set to the beam mode control factor .PHI./.omega. of 3.2 for use as an aperture for controlling the single mode.
In this conventional arrangement, the adjustment of the resonator mirrors is comparatively simple because the beam mode is controlled at one place. However, since the laser beam 2 path is also controlled at one place, the optical path 24 of the laser beam 2 varies within a range where the aperture control is loose, leading to a inability to maintain the pointing stability of the laser beam 2.
The conventional laser oscillating apparatus arranged as described above will not have stabilized laser beam pointing.